3 ms·

Nice, that's a cool example. But that's not fundamental to the concept of flow typing, it's just a mypy limitation.

planede 4y ago
And a typescript limitation? https://www.typescriptlang.org/play?#code/MYGwhgzhAEDC0G8BQ1XQB4C5oDsCuAtgEYCmATtAD654gjQC80ADANxIC+SSAZnjsAAuASwD2OaD1GiAFMGywAlNgBuo4QBNEKNMAB06RjTrsuvfkLESiYMnIXKaxctrTRhPaHIOMGTfHSKrm5oZCSCeGQSbDqoXG5SssCK7G5hEVHQ+uim3CDhGNj4zhRMNnY4JADucDKKKdzA4hCi+XogogDmMugpQA https://www.typescriptlang.org/play?#code/MYGwhgzhAEDC0G8BQ1...
- staticassertion 4y ago
  Yes, it seems that the issue is essentially one of binding the type to a scope but then mutating within that scope. The type system could just "look" for those mutations, or the language could add mutability to the type system.
  - planede 4y ago
    > The type system could just "look" for those mutations That's a very big "just". Typescript and mypy don't look through function call boundaries, doing so would open a lot of problems, I believe. > or the language could add mutability to the type system. I agree there, the core issue is that the call to `foo` leaks a an object where the member is rebindable through assignment, but the "flow typing" assumes that doesn't happen. Ideally such a call should be a barrier for flow typing, unless there was a way to specify functions that don't modify their parameter this way (readonly parameters?). A similar problem but with a global instead: https://www.typescriptlang.org/play?#code/DYUwLgBAlhBcEDsCuBbARiAThAPopwwEAvBACwBMA3AFA0BmSCAxmFAPYIT3vsAUASngA3dlAAmEAN40Ic6CXyFaAXzqMWbThDQBDTIPjJ0WabPlR6EPjGJ2lwAWfkuImcEkxcADLRdqXHn4BP3l3ME8uKFU6UEgADyNUDGxSPQMQumZOAGd2UAA6YHYAcz54kKA https://www.typescriptlang.org/play?#code/DYUwLgBAlhBcEDsCuB... This should be handled differently. Every function should be assumed to modify globals, unless they can be marked to be pure.
    - staticassertion 4y ago
      Yeah, that is definitely a big "just" - it's basically whole program type inference, which I expect to be very messy in Python, if not intractable. > unless there was a way to specify functions that don't modify their parameter this way (readonly parameters?). Right, so in a language with typed mutability (ie: Rust) or total immutability flow typing would be a lot easier to implement without these footguns. Python is such an insane language, I'm not surprised to see that you can subvert the type checker like this.

And a typescript limitation? https://www.typescriptlang.org/play?#code/MYGwhgzhAEDC0G8BQ1XQB4C5oDsCuAtgEYCmATtAD654gjQC80ADANxIC+SSAZnjsAAuASwD2OaD1GiAFMGywAlNgBuo4QBNEKNMAB06RjTrsuvfkLESiYMnIXKaxctrTRhPaHIOMGTfHSKrm5oZCSCeGQSbDqoXG5SssCK7G5hEVHQ+uim3CDhGNj4zhRMNnY4JADucDKKKdzA4hCi+XogogDmMugpQA https://www.typescriptlang.org/play?#code/MYGwhgzhAEDC0G8BQ1...

staticassertion 4y ago
Yes, it seems that the issue is essentially one of binding the type to a scope but then mutating within that scope. The type system could just "look" for those mutations, or the language could add mutability to the type system.
- planede 4y ago
  > The type system could just "look" for those mutations That's a very big "just". Typescript and mypy don't look through function call boundaries, doing so would open a lot of problems, I believe. > or the language could add mutability to the type system. I agree there, the core issue is that the call to `foo` leaks a an object where the member is rebindable through assignment, but the "flow typing" assumes that doesn't happen. Ideally such a call should be a barrier for flow typing, unless there was a way to specify functions that don't modify their parameter this way (readonly parameters?). A similar problem but with a global instead: https://www.typescriptlang.org/play?#code/DYUwLgBAlhBcEDsCuBbARiAThAPopwwEAvBACwBMA3AFA0BmSCAxmFAPYIT3vsAUASngA3dlAAmEAN40Ic6CXyFaAXzqMWbThDQBDTIPjJ0WabPlR6EPjGJ2lwAWfkuImcEkxcADLRdqXHn4BP3l3ME8uKFU6UEgADyNUDGxSPQMQumZOAGd2UAA6YHYAcz54kKA https://www.typescriptlang.org/play?#code/DYUwLgBAlhBcEDsCuB... This should be handled differently. Every function should be assumed to modify globals, unless they can be marked to be pure.
  - staticassertion 4y ago
    Yeah, that is definitely a big "just" - it's basically whole program type inference, which I expect to be very messy in Python, if not intractable. > unless there was a way to specify functions that don't modify their parameter this way (readonly parameters?). Right, so in a language with typed mutability (ie: Rust) or total immutability flow typing would be a lot easier to implement without these footguns. Python is such an insane language, I'm not surprised to see that you can subvert the type checker like this.